In recent years, the processes of the manufacture of semiconductor devices such as Large Scale Integrations (LSIs) and memories have become complicated, and semiconductor substrates have been increased in size and have become very expensive. For this reason, it is very important to improve the yield of the semiconductor device manufacture. That is, it is very important to use one substrate as efficiently as possible. To realize this, there are cases where in the ion implantation process among a plurality of semiconductor device manufacturing processes, the distribution of the amount of dose by ion implantation is intentionally made nonuniform within the surface of the substrate. As described above, there is a strong demand for the correction of the semiconductor device property in a specific region in a semiconductor device formed within the surface of the substrate.
For example, in the manufacture of a semiconductor device, in the process preceding ion implantation (e.g. a thin film formation process by vacuum deposition or Chemical Vapor Deposition (CVD), or an etching process), to uniformly perform processing within the surface of the substrate, in many cases, processing is performed with the substrate being rotated. However, even by that, processing cannot completely uniformly be performed, and as to property variation of the semiconductor device formed within the surface of the substrate, there is a tendency for variation to occur between a circular region and a peripheral region surrounding it.
To cite a more concrete example, the thickness of a gate oxide film formed on the substrate in the process preceding ion implantation tends to vary between a circular region and a peripheral region surrounding it within the surface of the substrate. If this is left as it is, in the manufacturing process in which a multiplicity of field-effect transistors are formed on a semiconductor substrate, variation occurs in the property of the field-effect transistors and the yield decreases.
Therefore, it is important to make it possible to form a circular implantation region and a peripheral implantation region surrounding it and the dose amount of which is different from that of the circular implantation region within the surface of the substrate in the ion implantation process. By thus changing the dose amount distribution, it is made possible to correct the variation in semiconductor device (e.g. field-effect transistor) property between the circular region and the peripheral region within the surface of the substrate and improve yield.
As a method capable of performing the ion implantation as described above, a method has been proposed in which as shown in FIG. 22, ion implantation is performed into a substrate 2 both by reciprocatively scanning an ion beam 4c in an X direction by an electric field or a magnetic field and by mechanically driving the substrate 2 in a Y direction intersecting the X direction. That is, this is an ion implantation method in which under a condition where the ion beam 4c is incident on the substrate 2, a circular implantation region and a peripheral implantation region surrounding it and the dose amount of which is different from that of the circular implantation region are formed within the surface of the substrate 2 both by stepwisely changing the scanning speed of the ion beam 4c or the driving speed of the substrate 2 and by step-rotating the substrate 2 about the central portion 2a thereof by 360/n degrees (n is the number of times of the implantation process), for example, in a direction indicated by the arrow J (see, for example, Japanese Patent Publication No. 4155327). The ion implantation using the step rotation of the substrate 2 as described above will be called step rotation implantation.
According to the above-described conventional ion implantation method, for example, it is possible to form three implantation regions A, B and A as shown in A of FIG. 23 after the first implantation process and form a plurality of implantation regions C to F as shown in B of FIG. 23 after the eighth (when n=8) implantation process. The central implantation region C is the circular implantation region, and the implantation regions D to F surrounding it are the peripheral implantation regions.
When the dose amount of one implantation process is d1 and d2, the dose amounts of the above-mentioned implantation regions A to F are mathematically as follows:
Implantation region A: dose amount d1 
Implantation region B: dose amount d2 
Implantation region C: dose amount 8d2 
Implantation region D: dose amount 2d1+6d2 
Implantation region E: dose amount 4d1+4d2 
Implantation region F: dose amount 6d1+2d2 